Targeted reagent injection for slag control from combustion of coals high in iron and/or calcium

ABSTRACT

Disclosed is a process that increases the output of a combustor fired with coal having high iron and/or calcium content, by reducing the tendency of slag to form on heat exchange surfaces and changing the nature of the slag to make it easier to remove. The process includes combusting a slag-forming coal, having high iron and/or calcium content, with an overall excess of oxygen; moving the resulting combustion gases though heat exchange equipment under conditions which cause cooling of slag formed by burning the fuel; and prior to contact with said heat exchange equipment, introducing aqueous aluminum trihydroxide in amounts and with droplet sizes and concentrations effective to decrease the rate of fouling, and preferably, increase the friability of the resulting slag. Desirably, the aluminum trihydroxide reagent is introduced in the form of an aqueous liquid and computational fluid dynamics is employed to determine flow rates and select reagent introduction rates, reagent introduction location(s), reagent concentration, reagent droplet size and/or reagent momentum. In a preferred aspect, the feed rate will up to about 6 pounds ATH per ton and preferably with up to about 2 pounds Mg(OH) 2  per ton of coal. A process is also provided for cleaning and maintaining cleanliness of a combustor.

PRIORITY CLAIM

This application claims priority to U.S. Provisional Patent Application No. 61/080,004, filed Jul. 11, 2008, the disclosure of which is incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to a process that increases the output of a combustor fired with coal having high iron and/or calcium content, by reducing the tendency of slag to form on heat exchange surfaces, changing the nature of the slag to make it easier to remove and actually removing slag.

Combustion of coal, like other fossil fuels, is invariably less efficient than desired and can be a source of pollution. Maintaining combustor operation at high efficiency and controlling the quality of the emissions is essential for maintaining the energy needed to power our economy while preserving the quality of the air we require for survival. Because efficiency and emissions are interrelated and some technological solutions have been shown to be competitive with each other, it has been difficult to achieve both. Economic operation of power plants and incinerators is in the public interest, and new technologies are essential to this effort.

Fuel selection plays an important role in mitigating some pollution problems, but it cannot eliminate them. Some coals, such as certain Appalachian and Illinois Basin bituminous coals, are important in many plants designed for coal where economics limits other options. The tendency to form slag and the properties of the slag for such high iron content coals have been a major concern of combustion engineers and plant operators for decades. There are a number of factors that impact the physical and chemical properties of slag. See, for example, Combustion Fossil Power, 1991, Joseph G. Singer, P. E., editor, Chapter 3, Combustion Engineering. However, as the industry stands today, there is a compromise between selection of low-cost coal and the actual economics of energy production where slagging becomes a problem. Slag accumulation is a problem that causes decreased heat transfer and often leads to long periods of downtime for cleaning.

An interrelated problem with coal is that large amounts of ash and fine particulates are formed that must be captured and disposed of. The art has used additives to control slag formation and properties, but the additives can stress the solids recovery systems employed in terms of sheer volume. Accordingly, optimum slag control has often been compromised because the solids recovery system could not effectively remove all of the solids necessary. This is especially a problem with older plants where increasing the solids collection capacity is not an option.

Making the problem more complex is the fact that coals react differently to additives as a function of their composition. As a general rule, there are no known formulae that make it possible to address all different coal compositions with suitable additives at effective levels that can be adequately handled by solids recovery equipment. The discovery of individual coal composition and additive regimens are highly sought after to assure that economical power can be supplied while generating sufficient revenues for effective pollution control.

There is a need for an improved process that more effectively controls slagging, especially with problem fuels, such as coals with sulfur contents that cause them to play an increased role in slagging and also those having high iron and/or calcium contents, to improve boiler efficiency and economics.

DISCLOSURE OF INVENTION

It is an object of the invention to provide an improved technology for slag control in combustors utilizing fuels tending toward the production of slag.

It is another object to provide a process to control slag from the combustion of coal with high iron and/or calcium contents while reducing chemical utilization.

It is another object to provide a process to remove slag from boiler heat exchange surfaces due to the combustion of coal with high iron and/or calcium contents while reducing chemical utilization.

A yet further but more specific object is to provide a process to more effectively control slag by decreasing the amount of downtime associated with slag removal.

It is a more specific object of some aspects of the invention to achieve the above objects while at the same time improving combustor efficiency.

These and other objects are achieved by the present invention in at least its preferred aspects which provides an improved process for slag control in combustors burning slag-forming coal with high iron and/or calcium content.

In one aspect, the invention provides a process for reducing slag cohesiveness and/or adhesiveness in a combustor, thereby decreasing the rate of fouling, comprising: combusting a slag-forming coal, having high iron and/or calcium content, with an overall excess of oxygen; moving the resulting combustion gases though heat exchange equipment under conditions which cause cooling of slag formed by burning the coal; and prior to contact with said heat exchange equipment, introducing aqueous aluminum trihydroxide in amounts and with droplet sizes and concentrations effective to decrease the rate of fouling, and preferably, increase the friability of the resulting slag.

In one preferred aspect, the aluminum trihydroxide reagent is introduced in the form of an aqueous liquid and computational fluid dynamics is employed to determine flow rates and select reagent introduction rates, reagent introduction location(s), reagent concentration, reagent droplet size and/or reagent momentum.

In another preferred aspect, magnesium hydroxide is introduced as an aqueous slurry along with the slurry of aluminum trihydroxide.

In another aspect the invention provides a process for cleaning furnace surfaces having a slag buildup, by introducing aqueous aluminum trihydroxide in amounts and with droplet sizes and concentrations effective to contact for fine particulates resulting from drying the slurry to contact existing slag deposits.

In another aspect, the invention provides a process a cleaning and maintenance of a combustor comprising a regimen of initial dosing of from about 3 to 6 pounds of ATH per ton of coal and about 1 to 2 pounds of Mg(OH)₂ per ton of coal for a time sufficient to reduce slag, followed by a reduced reducing the dosing of from about 10 to about 50% of the initial values for maintaining the combustor clean and operating efficiently.

Other preferred aspects and their advantages are set out in the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and its advantages will become more apparent when the following detailed description is read in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view of one embodiment of the invention.

FIG. 2 is a photograph of a slag sample obtained after operation for 24 hours of aluminum trihydroxide into a combustor operated on a high iron content coal as set out in Example 2 below.

DETAILED DESCRIPTION OF THE INVENTION

Reference will first be made to FIG. 1, which is a schematic view of one embodiment of the invention. FIG. 1 shows a large combustor 10 of the type used for producing steam for electrical power generation, process steam, heating or incineration. Coal is fed by burners 20 and 20 a and burned with air in a combustion zone 21. It is an advantage of the invention that coal that is high in iron (e.g., iron contents of greater than about 15%, e.g., from about 20 to 35%, based on the weight of the ash and expressed as Fe₂O₃) and/or calcium content (e.g., calcium contents of greater than 5%, e.g., from about 10 to 25%, based on the weight of the ash and expressed as CaO). It is also an advantage of the invention that slag can be effectively controlled even for coals having significant sulfur contents, e.g., above about 1% and in the range of from about 3 to about 5%. Here, and throughout this description, all parts and percentages are by weight.

Air for combustion, supplied by fan 22 and ductwork 24, is preferably preheated by a gas-to-gas heat exchangers (not shown) which transfer heat from ductwork (not shown) at the exit end of the combustor. Hot combustion gases rise and flow past heat exchangers 26, which transfer heat from the combustion gases to water for the generation of steam. Other heat exchangers, including an economizer (downstream and not shown) may also be provided according to the design of the particular boiler. Slag left untreated would tend to form on these heat exchanger surfaces, which are positioned within specific combustors based on design considerations important to individual locations. It is an advantage of the present invention that modeling techniques, such as computational fluid dynamics, are employed to initially direct treatment chemicals (especially, those identified as effective for particular types of coal according to the invention) to the optimum locations for reducing and/or controlling slag buildup and maintaining efficient operation of the boiler.

A series of suitable, preferably air assisted atomizing, nozzles in each of nozzle banks 30 and 30 a are provided for introducing aluminum trihydroxide alone or with magnesium hydroxide slurry from vessels 40 and 40 a respectively. Both the ATH and the magnesium hydroxide are preferably aqueous, as slurries and/or solutions as appropriate. Supply lines (e.g., 41) are shown as double lines in the drawing. Valves (e.g., 42) are represented by the common symbol

and temperature sensors (e.g., 44) are represented by the common symbol

Both valves 42 and temperature sensors 44 are connected to controller 46 via electrical leads (e.g., 48) shown in dotted lines. These valves, temperature sensors and leads are illustrative only, and the skilled worker using the principles outlined herein will place them strategically to provide appropriate control signals and responses. The controller 46 can be a general purpose digital computer programmed in accord with a predetermined control regimen with both feed forward and feedback features.

Aluminum trihydroxide (Al(OH)₃), which has been found effective according to the invention for greatly lessening the deposition of slag or cleaning deposited slag from troublesome coal types, is also known under other names such as ATH, aluminum hydroxide and hydrated alumina. Regardless of the form of aluminum trihydroxide raw material, it is preferred that it is mixed with water for introduction from tank 40 through associated lines 41, with or without chemical stabilizers, to concentrations suitable for storage and handling, e.g., at least about 25%, and preferably at least about 65%, solids by weight.

As will be described, the concentration and flow rates will be initially determined by modeling to assure that the proper amount of chemical is supplied to the correct location in the combustor in the correct physical form to achieve the desired results of reduced slagging and ease of clean up. For use in the process, it is diluted as determined, e.g., by computational fluid dynamics (CFD) to within the range of from about 0.1 to about 10%, more narrowly from about 1 to about 5%. When the aqueous aluminum trihydroxide contacts the hot gases in the combustor, it is believed to be reduced to very small particles, e.g., nano-sized particles, e.g., under 200 nanometers and preferably below about 100 nanometers. Median particle sizes of from 50 to about 150 nanometers are useful ranges for the process of the invention. To approach this size, it is important that the ATH be introduced with water. The small particles are believed to disrupt the normal crystalline or glass that forms the slag. Regardless of the mechanism involved it is a distinct advantage of the invention that the slag that does form is highly friable and breaks easily with brushing and can be crushed by hand.

It is a significant advantage of the invention that the friability of slag that is formed is increased, making it easier to remove. The invention also slows or eliminates the buildup of slag. Advantageously, at high doses, the invention can actually remove slag that has already formed. By the term “increase the friability of the slag” it is meant that the slag after treatment requires less force per unit area to crush than slag formed under the same conditions without the treatment. By the term “remove slag” it is meant that the weight of the slag adhering to boiler, particularly heat exchange, surfaces is reduced from initial values by the treatment of the invention. There are several additional and attendant advantages of the invention, including the reduction of SO₃ for high sulfur coals, the reduction of the pressure drop across heat exchange apparatus, the ability to use lower cost coal, lower CO generation, lower CO₂ generation due to increased fuel consumption, better heat transfer, less down time, higher throughput, cleaning on line, cleaner heat exchange surfaces, ability to clean the whole combustor, and the ability to run at all loads with greater efficiency.

The process for most coals works best with a combination of ATH and magnesium hydroxide. While some coals, e.g., with low silicate compositions can be burned with reduced problems attributed to slag, the use of magnesium hydroxide, at least initially, is preferred. The magnesium hydroxide reagent can preferably be prepared from brines containing calcium and other salts, usually from underground brine pools or seawater. Dolomitic lime is mixed with these brines to form calcium chloride solution and magnesium hydroxide which is precipitated and filtered out of the solution. This form of magnesium hydroxide can be mixed with water, with or without stabilizers, to concentrations suitable for storage and handling, e.g., from 25 to 65% solids by weight. For use in the process, it is diluted as determined by computational fluid dynamics (CFD) to within the range of from 0.1 to 10%, more narrowly from 1 to 5%. When it contacts the effluent in combustor, it is believed reduced to nano-sized particles, e.g., under 200 nanometers and preferably below about 100 nanometers. Median particle sizes of from 50 to about 150 nanometers are useful ranges for the process of the invention. Other forms of MgO can also be employed where necessary or desired, e.g., “light burn” or “caustic” can be employed where it is available in the desired particle size range.

To best achieve these effects, the invention will preferably take advantage of CFD to project initial flow rates and select initial reagent introduction rates, reagent introduction location(s), reagent concentration, reagent droplet size and reagent momentum. CFD is a well understood science, and it is utilized with full benefit in this case, where it is desired to supply a minimum amount of chemical for maximum effect.

It is noted as highly significant that the amount of chemical will be substoichiometric in terms of affecting the fusion point of the slag—often considered to be the controlling factor in slag control. According to the present invention, there is good evidence besides the relatively small amount of reagent employed that the results of the invention are due to a physical disruption of slag formation with possible boundary chemical and kinetic effects not explained by the literature.

Testing has shown that initial feed rates determined by CFD can be utilized with good effect and then adjusted based on observed results. As a guide to feed rates, the initial feed rate for the best economics for combustors operating similar to the one exemplified below can be up to about 6 pounds of ATH(as dry active ATH) or 8 pounds (as a 65-70% slurry) per ton of coal. For example, when added as a preferred 70% slurry, amounts of from about 1 to about 6 pounds of slurry will be effective (more narrowly, e.g., about 2 to about 3 pounds of slurry). It is preferred to also use up to about 2 pounds of Mg(OH)₂ slurry (at about 50-60% solids) per ton of coal. For example, when added as a preferred 60% slurry, amounts of from about 0.5 to about 2 pounds of Mg(OH)₂ slurry per ton of coal, e.g., from about 0.7 to about 1 pounds of Mg(OH)₂ slurry per ton of coal can be utilized. The slurries are diluted as necessary, typically to a solids concentration of from about 5% for smaller applications to about 35% or more.

The weight of the slag adhering to a combustor, particularly heat exchange, surfaces is effectively reduced from initial values by the treatment of the invention, especially when the ATH and Mg(OH)₂ are used at high concentrations within the above ranges, e.g., from about 3 to 6 pounds of ATH per ton of coal and about 1 to 2 pounds of Mg(OH)₂ per ton of coal. This ability to remove slag provides the ability to provide a cleaning and maintenance regimen wherein the initial dosing is as just mentioned for removing slag, with the dosing then reduced to from about 10 to about 50% of the initial values for maintaining the combustor clean and operating efficiently.

It is essential for optimum slag remediation according to the invention, that the correct initial concentrations, rates and introduction rates be calculated and employed for the effective physical form of aluminum trihydroxide, and preferably, optionally magnesium hydroxide, to be introduced into the hot combustion gases in chamber 20 to enable the chemical to be added with the desired effect. The implementation of CFD to the invention can be accomplished as set out in U.S. Pat. No. 7,162,960 to Smyrniotis, et al. Particulate removal equipment (not shown) can be employed to remove particulates prior to passing the effluent up the stack.

In another alternate form of the invention, combustion catalysts and or effluent treatment chemicals can be added to the fuel, combustion zone or otherwise as described, for example in U.S. Pat. No. 7,162,960 to Smyrniotis, et al.

The following examples are presented to further explain and illustrate the invention and are not to be taken as limiting in any regard. Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1

This example illustrates introduction of aluminum trihydroxide into a furnace burning 540 tons of coal per day. The coal is a blend of Illinois basin and Appalachian bituminous coals, giving the following analysis as combined:

Sample 1 2 3 Moisture, % 11.28 10.85 10.19 Ash, % 14.91 13.63 13.91 Volatile Matter, % 36.03 35.04 Fixed Carbon, % 39.49 40.86 Total, % 100 100 Sulfur, % 3.95 4.44 HHV, BTU/lb 10,742 10,730

For the test Al(OH)₃ (aluminum trihydroxide slurry or ATH for short) is fed as a 70% by weight aqueous slurry at a rate of 5 pounds slurry per ton of coal consumed from two banks of three air-cooled nozzles positioned on the wall opposite of two banks of pulverized coal burners—one bank at an elevation between the two burners and one bank at an elevation above the uppermost coal burners. The slurry is diluted to a concentration of 35 weight % ATH. The density of the ATH slurry before dilution is about 14 pounds/gallon, meaning that the feed rate is about 193 gallons per day (about 5 pounds per ton of coal) for ATH slurry.

Based on this test, it is estimated that an effective feed rate for this particular combustor will be from about 1 to about 6 pounds of ATH slurry per ton of coal, e.g., about 2 to about 3 pounds per ton.

EXAMPLE 2

This example illustrates the effect of introducing Mg(OH)₂ (magnesium hydroxide) into a furnace burning 540 tons of coal per day in addition to the aluminum trihydroxide fed in Example 1. The coal was a blend of Illinois basin and Appalachian bituminous coals, as illustrated in Example 1.

The magnesium hydroxide was fed as a slurry at 2 lbs of 50 to 60 weight % slurry per ton of coal consumed. Density of the magnesium hydroxide slurry was approximately 12 lbs/gallon. Therefore, the feed rate was about 90 gallons per day for the Mg(OH)₂ slurry. As before, we fed the aluminum trihydroxide slurry at about 5 pounds of slurry per ton of coal consumed. The density of the ATH was about 14 pounds/gallon, making the feed rate about 193 gallons per day for ATH.

Based on this test, we estimate optimal feed rate for the best economics for the this particular combustor to be about 0.5 to about 2 pounds Mg(OH)₂ slurry per ton of coal (e.g., about 1 pound per ton) plus from about 1 to about 6 pounds ATH slurry per ton (e.g., about 2 to about 3 pounds per ton). FIG. 2 is a photograph of a slag sample obtained after operation for 24 hours of ATH feed only. The slag was unexpectedly friable.

The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the invention. It is not intended to detail all of those obvious modifications and variations, which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the invention which is defined by the following claims. The claims are meant to cover the claimed components and steps in any sequence that is effective to meet the objectives there intended, unless the context specifically indicates the contrary. 

1. A process for reducing slag cohesiveness and/or adhesiveness in a combustor, thereby decreasing the rate of fouling, comprising: combusting a slag-forming coal, having high iron and/or calcium content, with an overall excess of oxygen; moving the resulting combustion gases though heat exchange equipment under conditions which cause cooling of slag formed by burning the fuel; and prior to contact with said heat exchange equipment, introducing aqueous aluminum trihydroxide in amounts and with droplet sizes and concentrations effective to decrease the rate of fouling by slag.
 2. A process according to claim 1, wherein the treatment is effective to increase the friability of resulting slag.
 3. A process according to claim 1, wherein, the aluminum trihydroxide reagent is introduced in the form of an aqueous liquid and computational fluid dynamics is employed to determine initial flow rates and select reagent introduction rates, reagent introduction location(s), reagent concentration, reagent droplet size and/or reagent momentum.
 4. A process according to claim 1, wherein the treatment further includes introducing magnesium hydroxide in amounts and with droplet sizes and concentrations effective to decrease the rate of fouling by slag.
 5. A process according to claim 1, wherein the treatment comprises introducing up to about 6 pounds of aluminum trihydroxide slurry per ton of coal and up to about 2 pounds of Mg(OH)₂ per ton of coal.
 6. A process for removing slag deposits in a combustor burning coal, comprising: introducing into hot combustion gases in the combustor, aqueous aluminum trihydroxide in amounts and with droplet sizes and concentrations effective to remove slag deposits.
 7. A process according to claim 6, wherein the treatment further includes introducing magnesium hydroxide in amounts and with droplet sizes and concentrations effective to decrease the rate of fouling by slag.
 8. A process according to claim 6, wherein the treatment comprises introducing up to about 6 pounds of aluminum trihydroxide slurry per ton of coal and up to about 2 pounds of Mg(OH)₂ per ton of coal.
 9. A process a cleaning and maintenance of a boiler comprising a regimen of initial dosing of from about 3 to 6 pounds of ATH per ton of coal and about 1 to 2 pounds of Mg(OH)₂ per ton of coal for a time sufficient to reduce slag, followed by a reduced reducing the dosing of from about 10 to about 50% of the initial values for maintaining the combustor clean and operating efficiently.
 10. A process according to claim 9, wherein the treatment further includes introducing magnesium hydroxide in amounts and with droplet sizes and concentrations effective to decrease the rate of fouling by slag. 